This invention relates to an improved method of making easy-open closures in cans, especially those made of sheet metal. More particularly, the invention is concerned with providing a better technique, when a closure has been at least partly defined by a weakening line including scoring-to-fracture, for consistently attaining a manually rupturable fluid-tight metal seal along the line of fracture.
In U.S. Pat. No. 3,881,630 issued May 6, 1975 in the names of Walter C. Lovell and Frederick G. J. Grise, there is disclosed a can end of sheet metal wherein the periphery of its easy-open closure is characterized by a fractured yet integral section. FIG. 9 of that patent, for instance, and related description disclose a variant form of closure involving a so-called "double indent" or "W-type" wall formation considered to have especial merit when practiced with tougher sheet metals, for example steel.
In U.S. Pat. No. 4,031,836, also issued in the names of Messrs. Lovell and Grise, there is disclosed a mechanism for dilating rim material of a can closure into frangible sealing relation with a can cover. This patent U.S. Pat. No. 4,031,836, as noted for instance in FIG. 6 thereof, embodies a flat swaging surface for enlarging the closure rim, and a coining surface for thereupon impacting the dilated rim material adjacent to a weakening line. Such an arrangement has been found generally satisfactory when operating upon softer container materials such as aluminum, but is not normally fully acceptable or even at times, suitably operative for sealing when dealing with tougher sheet metal that is less apt to flow radially upon impact. The difficulty encountered appears to be that with little or no dilated material suitably available to be worked into overlapping relation to the fractured weakening line, or with little control over the extent and precise disposition of the dilated material relative to that line, inadequate or insufficiently uniform closure strength can be provided by the prior art swaging to effectively seal the frangible steel joint. Assurance of predictability of a closure's opening upon the exertion of a reasonable, substantially uniform digital pressure to a can is, of course, highly desirable for consumer acceptance.